This invention relates to a coextruded, biaxially draw-oriented, multi-layer polyester film suitable for use as a base film for magnetic tapes.
Biaxially or multi-axially oriented polyester films, particularly polyethylene terephthalate, find extensive application in many technical areas such as the reprographics sector, as a dielectric for capacitors, as a base film for magnetic storage media such as audio, video and computer tapes, for magnetic cards and floppy discs. This is due to their superior properties such as tensile strength, tear propagation resistance, modulus of elasticity, transparency, chemical and thermal resistance.
These polyester films must meet specific requirements for the various areas of application which can be met by means of the polymer formulation or through certain process techniques during the manufacture of the film.
Polyester films which are to be processed into magnetic tape, for example, must have a low coefficient of friction and high abrasion resistance. For magnetic storage media with high information density and long playing times, thin base films with high tensile strength, especially good slip and abrasion resistance during long term stress, and good dimensional stability are required. Furthermore, the surface intended for the application of the information-carrying coating must be extremely smooth in order to eliminate interfering effects.
In order to reduce the surface roughness on the film surface which is intended for the magnetic coating, multi-layer films have been manufactured by techniques including coextrusion, such films being composed of an initial layer made of thermoplastic polymers without the addition of particles and having a smooth surface, and a second, thinner layer to which particles have been added in the usual manner.
U.S. Pat. No. 3,515,626, for example, describes films made from polyester polymers by means of coextrusion, which have at least one particle-filled layer. U.S. Pat. No. 3,983,285 describes coextruded polyester film having at least two layers having different physical properties and which contain soluble and insoluble precipitates. U.S. Pat. No. 3,958,064 describes multi-layer films having one smooth and one rough surface, which are achieved by the incorporation of two mutually insoluble polymers in the polyester layer. Finally, DE-OS No. 34 14 310 and DE-OS No. 34 14 347 describe co-extruded multi-layer base films for magnetic tape.
None of the previously available films, however, are able to simultaneously fulfill all requirements for the manufacture of magnetic storage media with higher information density, with respect to gauge, "machine direction" tensile strength, slip and abrasion resistance.
The object of this invention, therefore, was to develop a base film for magnetic tape, which is at the same time extremely thin, has a high MD tensile strength as shown by the modulus of elasticity (E-Modulus) in the machine direction (MD), and which furthermore has very good slip and abrasion resistance under extended stress.